Shigella, a major etiologic agent of diarrhea, dysentery, and mortality worldwide, causes disease by invading and disseminating through the colonic mucosa. Shigella sp. are CDC/NIAID Category B priority pathogens. After inducing their own entry into cells, bacteria move to the cell periphery by actin-based motility. At the cell periphery, they push out against the plasma membrane, forming cell extensions that are engulfed by uninfected adjacent cells, whereupon the cycle of cell-to-cell dissemination is repeated. Shigella and other intracellular pathogens enhance the process of infection by activating normal host cell signaling pathways. To activate these pathways, these microorganisms secrete into the host cell effector proteins that modulate the activity of specific host proteins. Whereas the molecular signaling events involved in S. flexneri entry into cells have been studied extensively, the signaling events involved in S. flexneri dissemination from one cell into an adjacent cell are poorly understood. In this application, we propose a detailed investigation of the molecular signaling events that occur during S. flexneri intercellular dissemination, using both targeted and genome-wide approaches. Our preliminary data indicate that the cellular diaphanous formin proteins mDia1 and mDia2, which function in the stress fiber formation pathway, are required for efficient dissemination through cell monolayers. Our data also indicate that the secreted S. flexneri proteins IpgB2, OspE1, and OspE2 are required for this process and that these proteins trigger partially redundant host signaling pathways that likely involve mDia1 and mDia2. In addition, our data indicate that IpgB2/OspE1/OspE2-independent mechanisms also contribute to dissemination. Our targeted approaches will test the hypothesis that IpgB2, OspE1, and OspE2 activate specific steps in the stress fiber formation pathway. Our genome-wide approach will examine the human genome for additional factors involved in intercellular dissemination. Our specific aims are: 1. Characterize the mechanisms of S. flexneri IpgB2 activation of the stress fiber formation pathway; 2. Characterize the roles of secreted S. flexneri effector proteins OspE1 and OspE2 in intercellular dissemination;and, 3. Identify and characterize other host factors required for S. flexneri intercellular spread using a genomewide human siRNA screen; Our approaches are designed to generate insights not only into the molecular signaling that is required for intercellular dissemination of S. flexneri, but also into fundamental mechanisms of eukaryotic cellular and intercellular processes.